Method and apparatus for aspirating and dispensing sample fluids

ABSTRACT

A method and apparatus for aspirating and dispensing a sample fluid. The apparatus includes a dilutor having a port coupled to a first port of a flow through pressure transducer and to a bleed valve. A second port of the flow through pressure transducer is coupled to a first port of a sample probe. The flow through pressure transducer provides transducer signals to a detector circuit. In response to the transducer signals provided thereto, the detector detects the occurrence or non-occurrence of a plurality of different events. The dilutor is operational to aspirate and dispense fluids such as air or liquid samples in order to provide accurate aspiration and dispensation of a sample fluid. The method includes the steps of first aspirating a predetermined amount of air with the dilutor, dispensing air while moving the sample probe towards the fluid sample, measuring the pressure in the probe and using this pressure value as a baseline, monitoring the pressure and detecting when the pressure changes, which is indicative of the probe entering the fluid sample, bleeding the pressure into the atmosphere while returning the dilutor to its home position, aspirating a predetermined amount of fluid to remove dilutor backlash, waiting a predetermined amount of time to finish bleeding the system, then aspirating the fluid sample with the dilutor. With such a method and apparatus, accurate aspirations/dispensations are provided, especially when small volumes are required.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of patent application Ser.No. 08/960,990, filed Oct. 30, 1997, now abandoned, which is adivisional application of patent application Ser. No. 08/501,806 filedJul. 13, 1995, now U.S. Pat. No. 5,750,881.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable

BACKGROUND OF THE INVENTION

As is known in the art, automated analyzers are used in clinicallaboratories to measure the various chemical constituents of bodyfluids, such as whole blood, blood serum, blood plasma, cerebral spinalfluid, urine, and the like obtained from patients. Automated analyzersreduce the number of trained technicians required to perform theanalyses in a clinical laboratory, improve the accuracy of the testingand reduce the cost per test.

Typically, an automated analyzer includes an automated fluid movingsystem which automatically aspirates a sample of body fluid from apatient's specimen container and dispenses the sample into a reactioncuvette. The fluid moving system typically includes a pipette whichaccomplishes the aspirate and dispensing functions under the control ofa robotic arm.

Chemical reagents, which are specific to the test being performed, aredisposed into the sample-containing cuvette thereby mixing the samplewith the chemical reagents. By examining the reaction products resultingfrom the mixing of the sample and reagents, the automated analyzerdetermines the concentration of the specific chemical constituent, forwhich the testing is being performed, in the patient's specimen. Uponcompletion of the test, the automated analyzer typically prints theresults of the test, including a sample identifier, a numerical resultof the test, and a range of values for the chemical constituent asmeasured by the test.

During an aspiration operation, the robotic arm, under command of asystem controller, positions the pipette above a specimen container andmoves the pipette into the container until the pipette reaches the fluidin the container. A syringe type pump is then typically operated to drawsample fluid from the specimen container into the pipette.

One problem that occurs with the fluid moving systems is thatoccasionally upon aspirating a sample, the sample pipette fails to beproperly disposed in the sample to be aspirated. In this case air,rather than a patient specimen, is drawn into the pipette. This preventsthe necessary sample volume of the fluid specimen from being aspiratedor from being completely dispensed into the reaction cuvette. If animproper sample volume of specimen is mixed with the reagents, anincorrect test result will typically be obtained.

Generally, when a clinician obtains an unusual test result, the test isrepeated and the new result compared to the previous result. If the tworesults do not agree to within a predetermined limit, the test must berepeated a second time in order to determine which of the previous tworesults is valid.

An additional prior art method includes aspirating air from the sampleprobe while the probe is being lowered toward the sample. Thisaspiration while moving the probe results in varying amounts of air inthe system when fluid is aspirated, which in turn results ininaccuracies for the aspiration/dispensation of the sample. Theseinaccuracies can be particularly troublesome when aspirating/dispensingsmall volumes (i.e. 10 ul to 200 ul volumes).

Thus, it would be desirable to provide an automated fluid sampleaspiration/dispensation device which detects physical contact between aprobe tip and a surface of a liquid to thus ensure that a fluid ratherthan air is drawn in to the sample probe.

BRIEF SUMMARY OF THE INVENTION

In accordance with the present invention, an apparatus for aspiratingand dispensing a sample fluid includes an air source having an outputport coupled to an input port of a flow through pressure transducer. Anoutput of the flow through pressure transducer is coupled to a sampleprobe which has a tip that contacts the sample fluid. With thisparticular arrangement an apparatus for detecting physical contactbetween the sample probe and a surface of a liquid sample is provided.The pressure transducer senses pressure changes which result from anumber of other events including but not limited to: (a) fluid leaks ina fluid path; (b) aspiration through the sample probe; (c) obstructionof a sample probe tip; and (d) attachment and detachment of a sample tipto a sample probe. The apparatus may also include a detector circuitcoupled to the transducer. In response to each of the above-identifiedevents, the flow through pressure transducer provides a differentialvoltage signal to the detector circuit.

In a surface detection mode of operation, the air source provides aconstant air flow through the pressure transducer and the sample probe,and probe tip while the sample probe is being lowered toward a surfaceof a fluid. Once the sample probe tip reaches the surface of the liquid,the pressure transducer senses a change in pressure of the air path inwhich the pressure transducer is disposed. In response to the pressurechange, the transducer provides transducer signals to the detectorcircuit. The detector circuit detects the signals provided thereto andprovides a control signal to a system controller.

The detector circuit may be provided with the capability to detectseveral events including but not limited to: leaks; fluid level;aspirate integrity; clots; tip presence and pump servo integrity. Eachof the events result in identifying signals being provided to a systemcontroller for control of the air pump and the sample probe.

In detecting the position of a surface of a fluid, a sample probe ismoved toward a fluid surface and when contact is made, a change inpressure in the air path inline with the flow through pressuretransducer provides a pressure transducer signal representative of thecontact, further permitting determination and the location of the fluidsurface.

Leaks are detected in a fluid path of an aspirate and dispense apparatususing the same apparatus operated to occlude the sample probe tip byinserting the sample probe tip into a sample fluid and sensing thesignal provided by the pressure transducer. A sensed pressure below anormalized pressure indicates leaks in the fluid path of the aspirateand dispense apparatus. If a leak exists, the pressure will not rise tothe normal level each time. The normal pressure can be established byplacing a calibration tip having no opening for aspiration onto thesample probe body.

The detector circuit also detects when a sample probe tip is beingcoupled to a sample probe at a tip loader and removal of the sampleprobe tip at a tip dispense position by the increase in pressure whenthe smaller tip opening is placed over the sample probe. The detectorcircuit also detects when a sample probe tip is occluded by anobstruction during an aspirate or dispense operation. The occlusion mustbe severe enough to trigger a predetermined pressure change in thepressure transducer. The detector circuit also provides an indicationwhether a system bleed valve is closed or open. The detector circuitalso evaluates the pump servo integrity by comparing the air pumpvoltage with a voltage of the pressure transducer in the tubing anddetermining whether the voltage relationship is within predeterminedlimits. The detector circuit also determines aspirate integrity byverifying that an air aspiration results in a pressure change withinpredetermined limits.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

This invention is pointed out with particularity in the appended claims.The above and further advantages of this invention may be betterunderstood by referring to the following description taken inconjunction with the accompanying drawing, in which:

FIG. 1 is a block diagram of an automated fluid sampleaspiration/dispensation apparatus;

FIG. 2 is a diagrammatical view of an automated fluid sampleaspiration/dispensation apparatus;

FIG. 3 is a block diagram of a detector system;

FIG. 4 is a schematic diagram of detector circuits for variousfunctions;

FIG. 5 is a diagram of a further embodiment of an automated fluid sampleaspiration/dispensation apparatus; and

FIG. 6 is a flow chart of a method using the embodiment of FIG. 5.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIG. 1, aspirating and dispensing apparatus includes aconstant air source 12 having an output port 12a coupled through a twoway bleed valve 14 to a first port 16a of a three way pump valve 16.Bleed valve 14 has a vent 14' which is controlled by bleed valve 14 tobe open or closed as described below.

The constant air source 12 should be of a type capable of providing aconstant air flow at a predetermined rate and pressure to the pump valve16. This rate and pressure is fairly low and depends on overall systemparameters.

A second port 16b of pump valve 16 is coupled to a first input port 18aof a T-junction 18 and a third port 16c of the valve 16 is coupled to avent. A second port 18b of the T-junction connector 18 is coupled to asample probe dilutor 20 which may be provided for example as a syringeor pumped dilutor source.

A third port 18c of T-junction connector 18 is coupled to a flow throughpressure transducer 22 at a first port 22a. A second port 22b of thetransducer 22 is coupled to a sample probe 24 which may, for example, beprovided as a pipette tube holder. Thus, the pressure transducer 22 islocated in-line with a fluid conduit between the air source 12 and thesample probe 24.

The pressure transducer 22 is preferably located proximate the sampleprobe 24 to thus improve the signal to noise ratio of the pressuremeasurement. The sample probe 24 is controlled by a robot arm 23 to moveto and/or from a cuvette 32 to aspirate or dispense in an automatedassay system or to/from a tip stations 25 and test tube 27. In responseto fluid flow through the pressure transducer 22 the transducer providesan electrical signal through a signal line 26 to a detector system 28.The detector circuit 28 receives input signals from the transducer 22and provides output signals to the air source 12 and to a microprocessorbased controller 33 via a microprocessor bus 30.

The detector system 28 detects the occurrence or non-occurrence ofdifferent events throughout an analyzer cycle automated analyzer system.

In response to the input signals from the transducer 22, the detectorsystem 28 provides a plurality of functions to indicate by appropriateoutput signals when a distal end of the sample probe 24, typicallyhaving a pipette tip, physically contacts a fluid sample 31 disposed intube 27 or cuvette 32 into which the sample probe 24 is lowered by arm23.

FIG. 2 shows in more detail the system of FIG. 1. As shown therein anaspirating and dispensing apparatus includes constant air source whichhere includes an air pump 70 coupled to an accumulator 72 having avessel in which air provided from the air pump 70 is stored at aparticular pressure such that a supply of air at a constant, lowpressure is immediately available at an output port 72a of theaccumulator. In this particular embodiment, the accumulator 72 isprovided as a coil of tubing 73 which acts to regulate the pressure andvariable flow rates and is diminished according to the needed slowregulation in pressure measurement. A three port connecting member 74disposed between the coil 73 and the air pump 70 has a first portcoupled to the output port of the air pump 70 and a second port coupledto a first port of the coil 73. A third port of the connecting member 74provides a vent port to which is coupled a vent tube 76.

To ensure proper operation of the aspirating and dispensing apparatus,the air pump 70 provides a relatively low air flow at the output port72a of the accumulator 72. To provide such an air flow, connectingmember 74 vents a portion of the flow from air pump 70.

The vent tube 76 may preferably be provided as part of the coil 73 (e.g.provided on an inside portion the accumulator coil 73). The vent 76establishes an upper pressure limit to which the pump 70 will be exposedeven in the case of complete occlusion of the sample probe.

The accumulator 72 may also be implemented using other techniques wellknown to those of ordinary skill in the art.

The accumulator output port 72a is coupled through a bleed valve 78 to acommon port 80a of pump valve 80 corresponding to pump valve 16. Thepump valve 80 also includes a normally open port 80b to the sample probeand a normally closed port 80c to a vent 80d. The pump valve 80 iscontrolled by a controller 94.

The port 80b of pump valve 80 is coupled to a first port 82a of a threeport connecting member 82. A second port 82b of the three portconnecting member 82 is coupled to a dilutor 84. The dilutor 84 may beprovided, for example as a syringe pump, in which the movement of apiston 86 in a first direction forces fluid from a housing 88 whilemovement of the piston 86 in a second opposite direction pulls fluidinto the housing 88 through port 82b.

A shaft 90 couples the piston 86 to a linear stepper motor 92. Inresponse to signals received from controller 94, the stepper motor 92drives the piston 86 in first and second opposite directions within thehousing 88. In a preferred embodiment, the controller 94 is provided asa microprocessor based controller.

A third port 82c of the three port connecting member 82 is connected toa tube 96 having an inner diameter which fits the port 82c, sealing theconnection.

A pressure transducer 98 has a first port 98a coupled to a second end ofthe tube 96 and a second port 98b coupled to a first end of a typicallyresilient tube 100. A second end of the tube 100 is coupled to a firstport of a sample probe 102. Thus, the connecting member 82 and tubes 96,100, and pressure transducer 98 provide a fluid path between the sampleprobe 102 and the pump valve 80, and dilutor 84.

The pressure transducer 98 is here provided as a flow through pressuretransducer of the type manufactured by the Micro Switch Division ofHoneywell Corporation and identified as a 26PC Series pressuretransducer and more particularly as part number 26PC BFG 6G. Thesensitivity of the transducer 98 corresponds to about 10 mV/PSI ofpressure difference. Other flow through pressure transducers havingsuitable fluid and electrical characteristics may also be used.

To facilitate connecting of the transducer ports 98a, 98b to therespective ones of the tubes 96, 100 with substantially differentdiameters, each of the ports 98a, 98b has coupled thereto a mating tube101. The mating tubes 101 are provided from a relatively flexiblematerial having a relatively high elasticity characteristic and anon-stretched diameter selected to accept the outside diameter of thetubes 96, 100 with a slight interference fit.

The sample probe 102 includes a probe body 106 having a channel 110between a first fluid port 106a to which the system tubing 100 iscoupled and having a second fluid port 106b to which a sample probe tip108 is coupled. In this particular embodiment, the sample probe tip 108is provided as a disposable sample probe tip which is removably coupledto the sample probe body 106. It should be appreciated, however, that insome applications it may be desirable to provide the sample probe tip asa non-disposable plastic tip which is permanently secured to the sampleprobe body 106.

The tube 100 which couples the transducer 98 to the sample probe 102 ishere provided having a length typically of about nine and one-halfinches. It is desirable to minimize the distance between the sampleprobe 102 and the pressure transducer 98. In some applications, it maybe desirable or even necessary to place the pressure transducer 98closer than nine and one-half inches from the sample probe 102 and asclose as possible to the sample probe 102.

In applications in which it is desirable to maximize sensitivity of theapparatus 66 to small changes in pressure, for example, it would bedesirable to directly mate the transducer 98 to the sample probe 102. Inpractical applications, however, it is often not possible due the sizeof circuit components and available packaging space to achieve thisgoal. Thus, as trade-off, the pressure transducer 98 should be coupledto the sample probe 102 via a tube which minimizes the length of thefluid path between the transducer 98 and the sample probe 102.

For this purpose, the pressure transducer 98 may be disposed on aprinted circuit board (PCB) coupled to the sample probe 102 or asmentioned above, if space permits the pressure transducer may bedirectly disposed on the sample probe 102.

In this particular embodiment, the flow through pressure transducer 98has a pair of electrical terminals 98c, 98d one of which corresponds toa positive output terminal and one of which corresponds to a negativeoutput terminal of the transducer 98. The transducer 98 provides adifferential output voltage on the output terminals 98c, 98drepresentative of the pressure difference between the pressure in thesample probe tip and an ambient atmospheric pressure.

The transducer 98 is electrically coupled through lines 111 to adetector circuit 112 at a pair of input terminals 112a, 112b. Thedetector circuit 112 receives input signals from the pressure transducer98 and provides at its output terminals output signals to controller 94and to the air pump 70.

In operation, prior to aspirating a sample fluid from a tube 27 orcuvette 32, the vent port 80c of pump valve 80 is initially closed andthe common and sample probe ports 80a, 80b are initially open. Also, thevent port of the bleed valve 78 is closed and the piston 86 ispositioned so that no fluid is inside the housing 88. The air pump 70 isthen turned on, forcing air through a fluid path which leads to thesample probe tip 108. Thus, air is forced out of the sample probe tip ata predetermined rate which creates a predetermined pressure measured bypressure transducer 98.

The sample probe 102 is moved toward a region in which fluid is expectedto be contacted, such as in the tube 27. When the distal end 108a ofsample probe tip 108 initially contacts fluid, the distal end 108a isoccluded by the fluid. This results in the fluid conduit coupled betweenair pump 70 and the sample probe tip 108, including fluid lines 96, 100,being pressurized. The pressure transducer 98 senses the increasedpressure level and provides a transducer signal to the detector circuit112.

The detector circuit 112 then provides a control signal to thecontroller 94 which stops the sample probe from being lowered further orbeyond a preset point into the fluid sample. The controller 94 providescontrol signals to open the vent port of the bleed valve 78 to thusde-pressurize the fluid path between the air pump 70 and sample probe102 including the fluid path in which the pressure transducer 98 isdisposed.

After the fluid lines have been de-pressurized, the controller 94 closesthe vent port of the bleed valve 78. De-pressurizing the fluid pathbetween the dilutor 84 and the connecting member 82 prior to moving thepiston 86 improves the ability of the system to accurately determine theaspirate and dispense fluid volumes. If the fluid path between dilutor84 and connecting member 82 were pressurized when the piston 86 began tomove, the dilutor 84 would initially be forced to overcome the pressurebuilt up in the fluid path. Thus, rather than aspirating fluid inresponse to movement of piston 86, pressure in the fluid path betweenthe dilutor 84 and sample probe 102 would be equalized with the pressurein the dilutor, otherwise it is relatively difficult to preciselydetermine the amount of fluid which was drawn in by the dilutor 84.However, by opening and then closing the bleed valve 78, the pressure inthe fluid line is set to atmospheric pressure. Thus, fluid can beimmediately drawn into the sample probe tip 108 in response to operationof the dilutor 84.

The apparatus also detects leaks in the fluid paths. To detect leaks,the distal end 108a of the sample probe tip 108 is completely occludedand the tubing is pressurized by turning on the pump 70.

The distal end 108a of the probe tip 108 is occluded and the pump 70 isleft on. The pressure in the fluid path between sample probe 102 andconnecting member 82 is thus allowed to rise to a predetermined limitestablished during a calibration routine. If no leaks exist, then thepressure will rise to substantially the same calibration level each timethe sample probe is occluded. If a leak exists, however, the pressurewill not rise to substantially the same level each time.

For each system, a calibration routine will be performed whereby the tipis occluded and the pressure to which fluid in the fluid paths rise isdetermined. The tip's distal end 108a may be occluded, for example, byplacing a calibration tip onto the sample probe body 106. Such acalibration tip would be provided having an opening in one end thereofto be attached to the sample probe port 106b and no opening in thesecond end thereof.

The system controller 94 would then perform a sample probe calibrationroutine to establish a threshold pressure and voltage.

Referring now to FIG. 3, detector circuit 112 is shown to include afluid pressure transducer 122 (corresponding to transducer 98 and 22)having a pair of fluid ports 122a, 122b and a pair of electrical signalterminals 122c, 122d on which a differential electrical signal iscoupled to an amplifier circuit 124. The transducer 122 detects pressurechanges which result in the fluid path due to the occurrence ofparticular events. For example, the transducer 122 senses pressurechanges which result from a number of events including, but not limitedto some or all of the following: (a) fluid leaks in a fluid path; (b)contact between a sample probe tip and a surface of a fluid; (c)aspiration of air through a sample probe; (d) obstruction of a sampleprobe tip; and (e) attachment and detachment of a sample tip to a sampleprobe.

In response to each of these events, the flow through pressuretransducer 122 provides a corresponding differential voltage signal toan amplifier circuit 124 at input terminals 124a, 124b. The amplifiercircuit 124 receives the differential signal fed thereto from thepressure transducer 122 and provides an amplified single ended outputsignal at an output terminal 124c thereof. The amplified output signalis fed to an input terminal 126a of a signal conditioner and pumpcontrol circuit 126.

A plurality of event detector circuits 128, 130, 132, 134, and 136 arecoupled to an output terminal 126b of the signal conditioner and pumpcontrol circuit 126 to receive a pressure signal; and a pump servointegrity circuit 138 is coupled to an output terminal 126c of signalconditioner and pump control circuit 126. While each of the circuits128, 130, 132, 134, and 136 will be described further below, in generaleach of the circuits 128, 130, 132, 134, and 136 receives an inputsignal from signal conditioner and pump control circuit 126 atrespective input terminals 128a, 130a, 132a, 134a, and 136a thereof andcompares the signal level of the input signal to one or more thresholdsignal levels internally generated. Each of the circuits 128, 130, 132,134, 136, and 138 may be provided having different threshold signallevels. Circuit 126 may be software implemented or otherwise as may beeffective.

In response to the input signal having a signal level either greater orless than the threshold signal levels, each of the circuits 128, 130,132, 134, 136, and 138 provide representative output signals at theoutput terminal thereof. Each of the output terminals 128b, 130b, 132b,134b, 136b, and 138b are coupled to controller 94 described above inconjunction with FIG. 2.

The output signals indicate whether or not a particular event occurredor the status of the aspirate-dispense apparatus. It should be notedthat each of the circuits 128, 130, 132, 134, 136, and 138 and 126 maybe implemented via a programmed microprocessor or alternatively may beimplemented via comparator circuits.

An output terminal 126d of the signal conditioner and pump controlcircuit 126 is coupled to an air pump 140 which corresponds to pump 70.

The leak detector circuit 128 receives the signal on line 126b anddetects whether any leaks exist in the fluid paths of the apparatus(FIG. 2). When operating in a leak detection mode, the controller 94(FIG. 2) pressurizes the fluid paths in the apparatus 66. Leak detectorcircuit 128 measures the signal level of the signal on line 126b and inresponse to the signal level, detector circuit 128 provides a signal tocontroller 94. The signal level of the line 128b signal indicates tocontroller 94 whether or not a leak exists in the fluid paths ofapparatus 66.

Fluid level detector circuit 130 detects when the distal end 108a of thesample probe tip 108 physically contacts and is inserted into a samplefluid.

Aspirate integrity detect circuit 132 detects whether or not pump valve80 is operating correctly. After a tip is placed on the sample probe,the sample probe port 80b of pump valve 80 (FIG. 2) is closed and air isaspirated. This should result in a pressure change to a predeterminedlevel. If there is a leak in the tubing or the sample probe port 80b didnot close, then the pressure change will not reach the proper level.Thus, the aspirate integrity detect circuit 132 indicates whether or notthe pump valve 80 has worked correctly.

Clot detector circuit 134 detects whether or not the distal end 108a ofthe sample probe tip 108 was occluded during aspirate and dispenseoperations.

In those applications where probe tip 108 is provided as a plasticdisposable probe tip, tip detector circuit 136 detects when the probetip is coupled to and decoupled from the sample probe body 106 based ona change in pressure to predetermined levels in each case.

Pump servo integrity circuit 138 monitors the voltage signal used toservo the air pump 140 and determines whether or not an appropriateservo voltage is being applied to the pump 140. An incorrect voltagewould indicate an error in condition such as a blocked flow path.

By examining detector signals provided from detector circuits 128, 130,132, 134, 136, and 138, a number of failures in the aspirate anddispense apparatus of FIG. 2 can be detected. For example, a failedpressure transducer, a failed air pump, or a bleed valve stuck in theopen position (i.e. always bleeding) may be detected by examiningsignals on lines 130b, 136b, and 138b.

A bleed valve stuck in the closed position (i.e. never bleeding) may bedetected by examining the line 130b signal.

Similarly, the 132b signal may be examined to detect if the sample probeport of the pump valve is stuck in the open position so as tocontinuously provide air from the air pump 70 (FIG. 2) to the sampleprobe 102. A pump valve stuck in the closed position such that the pumpvalve fails to provide air to the sample probe 102 may be detected byexamining the 130b and 138b signals.

A leak in the tubing large enough to affect dispense performance or thelevel sense operation may be detected by examining the line 138b, 132b,and 128b signals.

It should be noted that each of the event detector circuits 128, 130,132, 134, 136, and 138 compares the respective input signal fed theretoto internally generated threshold voltage levels to determine theoccurrence or non-occurrence of particular events. In response to thecompare operations, each of the event detector circuits 128, 130, 132,134, 136, and 138 provides an appropriate output signal to controller94.

Referring now to FIG. 4, the details of the signal conditioner and pumpcontrol circuit 126 are shown.

The input to the signal conditioner and pump control circuit is coupledthrough a resistor 155 to an inverting input of an inverting amplifier160. The inverting amplifier 160 provides the line 126b signal to thedetectors 128, 130, 132, 134, and 136.

A resistor R1 and capacitor C1 are coupled in a negative feed-back pathas shown between the output terminal 160c of the inverting amplifier 160and the inverting input terminal of the amplifier 160. A non-invertinginput of the inverting amplifier 160 is coupled to a first terminal 162aof a sample and hold circuit 162.

A charge storing capacitor 164 for the hold function is coupled betweena second terminal 162b of the sample and hold circuit 162 and ground. Athird terminal 162c of the sample and hold circuit 162 is coupled to anoutput terminal of a second inverting amplifier 166 and a fourthterminal 162d of the sample and hold circuit 162 is coupled to thesystem controller 94.

The non-inverting input of the second inverting amplifier 166 is coupledto ground and the inverting input of the amplifier 166 is coupledthrough a resistor R2 to the output terminal 160c of the first invertingamplifier 160. A feedback capacitor C2 is coupled between the output andthe inverting input of the amplifier 166.

The output terminal 160c of the first amplifier 160 is also coupledthrough a resistor R3 to an inverting input of a third invertingamplifier 170. The non-inverting input of the inverting amplifier 170 iscoupled to a reference voltage 171 through a voltage divider network 172having resistors R4, R5 selected in conjunction with the voltage levelof the reference voltage 171 such that a predetermined threshold voltageis provided to the non-inverting input terminal 170b of the invertingamplifier 170.

A capacitor C3 is coupled between the output terminal 170c and invertinginput of amplifier 170.

The output terminal 170c of the third inverting amplifier 170 is coupledto a first terminal 174a of a sample and hold circuit 174. A chargingcapacitor 176 is connected between a second terminal 174b of the sampleand hold circuit 174 and ground. A third terminal 174c of the sample andhold circuit 174 is coupled through a resistor R6 to an input terminal178c of a voltage regulator circuit 178 and a fourth terminal 174d ofthe sample and hold circuit 174 is coupled to the system controller 94.The system controller provides a control signal to the sample and holdcircuit causing it to operate in either a sample mode or a hold mode.

The voltage regulator 178 has a voltage input terminal 178a coupled to areference voltage source 179. A voltage output terminal 178b ofregulator 178 is coupled through a resistor 188 to control pump 140. Azener diode 184 is coupled between the input 178c and ground clampingthe input to not exceed a predetermined voltage.

A resistor 186 is coupled between node 180 and the anode of the zenerdiode 184 as shown. A switch 192 is coupled between a second terminal ofpump 140 (corresponding to air pump 70) and ground. In response to afirst control signal from controller 94, the switch 192 is madeconducting, activating the air pump 140.

The sample and hold circuit 162 establishes a reference or normalizedvoltage level for the signal on line 126b corresponding to a referenceor normalized pressure level in the pressure transducer 122.

The sample and hold circuit 162 is placed in sample mode by thecontroller 94 in which it connects a signal path between the output ofamplifier 166 and the non-inverting input of amplifier 160. Amplifier166 provides an output signal to sample and hold input terminal 162c.

Amplifier 166 provides a bias signal at its output that is applied tothe non-inverting input of amplifier 160 via sample and hold circuit 162until the signal provided at the output of amplifier 160 is driven to avoltage level corresponding to ground. At this point, controller 94provides a second control signal to sample and hold terminal 162d whichplaces the sample and hold circuit in the hold mode. The voltage levelof the sample and hold circuit is thus setting a value which causes theline 126b output to be zero for what ever pressure is sensed. Thereafterthe voltage level of the signal on line 126b is representative ofrelative pressure changes detected by the pressure transducer.

System operation:

(1) a system cycle begins with the sample probe 102 (FIG. 2) without atip. A control signal from the controller 94 (FIG. 2) biases the switch192 into its non conduction state thus decoupling the air pump 140 fromground and thereby turning off the air pump 140. With air pump 140 off,no pressure exists in the fluid path in which the flow through pressuretransducer 122 is disposed. Thus, the pressure transducer 122 provides adifferential output signal corresponding to zero pressure to the inputterminals of the amplifier 124 (FIG. 3). Also, with the pump 140 turnedoff, the voltage regulator 178 and zener diode 184 maintain the voltageat line 178b at a set voltage level. Furthermore, the output terminal ofamplifier 170 provides a voltage level corresponding to the railvoltage.

(2) The controller 94 then provides a control signal to the sample andhold circuit 162. In response to the control signal, the sample and holdcircuit 162 drives output 126b to zero volts.

(3) The controller 94 then provides a control signal to turn the pumpvalve 80 on (FIG. 2) and also provides a second control signal to biasthe switch 192 into its conduction state thereby turning on the airpump. When the pump is initially turned on the voltage across the pumpis at a high voltage. When pump 140 is first turned on, amplifier 170servos the pump voltage so that line 126b is at the voltage at itsnoninverting input. Prior to the pump turn-on, the output of amplifieris at the positive rail driving current through resistor R6 and forcingline 178c to the zener diode voltage set by zener 184. This causes arapid spinning of pump 140. Over time, amplifier 170 servos the loopresulting in its output falling as line 126b increases with the build upof the pressure signal from transducer 122. At a desired pressurevoltage, the sample and hold circuit 174 is caused by controller 94 tohold that voltage for a cycle. Also, in response to the pump beingturned on the pressure in the system fluid lines rises rapidly.

(4) After a period of typically five hundred milli-seconds thecontroller 94 provides a control signal on line 174d to the control portof the second sample and hold circuit 174, thus placing the sample andhold circuit in the hold mode. The zeroing of output line 126b procedureof step "2" is repeated here as a very fast recalibration step.

(5) The controller 94 then measures the output of the integrity circuit138 to determine if the signal is within a predetermined voltage range.If the signal has a voltage level outside a predetermined voltage rangethen an error signal is generated by the controller 94 and processingstops. If the signal has a voltage level within a predetermined voltagerange then processing continues and the controller moves the sampleprobe 102 via robot arm 23 to a station 25 (FIG. 1) at which it may pickup a disposable probe tip.

(6) Line 126b is again zeroed as in step "2" and a fresh tip is placedon the probe. During placement of a probe tip on the probe body, the airin the line experiences a 15 brief transient as the tip is inserted.Controller 94 examines the signal on line 136b from detector 136 todetermine whether the signal corresponds to a preset level for apredetermined period of time to confirm tip placement. The pump filteris on and all values stay as set.

(7) The controller 94 turns off the pump valve 80. The air pump 140remains running.

(8) Line 126b is zeroed again according to step "2"

(9) Once the tip is coupled to the sample probe body, the controller 94engages the stepper motor 92 which draws the piston 86 into the cylinder88. Since the tip has not yet been disposed in a fluid, this results inair being aspirated into the fluid paths through the disposable sampleprobe tip. After the aspiration is complete, the controller determineswhether an error exists via the aspirate integrity detector circuit 130(FIG. 3) as follows

(10) The controller 94 turns the pump valve 80 on thus supplying airflow to the sample probe 102. The controller 94 then zeros line 126b.

(11) The controller 94 moves the sample probe via robot arm 23 to aposition in which the sample probe can access a sample tube 27 holding afluid sample.

(12) The sample probe is lowered in the direction of the fluid tube 27.Once the disposable probe tip reaches the fluid, the pressure transducer122 senses a change in pressure and provides a signal to the detectorcircuit 112. In response to the signal provided thereto from thepressure transducer 122, the detector circuit 130 generates a signal onoutput 130b and provides the signal to the controller.

(13) While monitoring line 130b, the controller 94 moves the disposabletip into the fluid sample to a predetermined penetration depth selectedto allow aspiration of the needed fluid.

(14) After the disposable tip is moved to the predetermined depth, thecontroller waits for a predetermined period of time, typically aboutfive hundred msec, and then examines the signal on line 128b provided byleak detect circuit 128 (FIG. 3) to determine if any leaks are presentin the system.

(15) The controller 94 provides control signals to turn on the bleedvalve and turn off the air pump as described above to normalize thefluid line in preparation for aspirating fluid.

(16) The controller 94 then turns off the pump valve 80 and

(17) engages the stepper motor 92 (FIG. 3) causing the dilutor toaspirate fluid into the sample probe tip 108. The controller 94 alsomonitors the line 134b signal provided by clot detector circuit 134 todetermine if the sample probe fluid path has been obstructed during theaspirate operation.

(18) If no clot detection occurs then the controller 94 moves the sampleprobe to a position in which a sample fluid may be dispensed into acuvette 32.

(19) The controller 94 provides a control signal causing the steppermotor to dispense the sample fluid into the cuvette 32. The controlleragain examines the line 134b signal to determine if the sample probefluid path has been obstructed during the dispense operation.

(20) After the dispense operation is complete the controller 94 movesthe probe body to eject the disposable tip. The signal on line 136bprovided by tip detector circuit 136 (FIG. 3) should be present a fewmsec. thereby indicating that the disposable tip is removed from thesample probe body.

A leak is detected by leak detection circuit 128 (FIG. 3) in thefollowing manner. As described above in conjunction with FIG. 4, thedisposable tip of the sample probe is moved toward the fluid sample withthe air pump on thus allowing the detection of the fluid sample level.Once the disposable tip is disposed in the fluid, the air pump remainson thereby letting pressure build up in the fluid lines.

The air pump provides the air at a flow rate which does not allow thepressure to rise to a level which causes a bubble in the sample fluid.Rather, pressure in the sample probe and fluid path leading theretobuilds to a static pressure. The pressure range of the static pressurewill be known from a calibration step to be described below.

The predetermined static pressure level corresponds to an equalizingpressure. Ideally, the pressure should build up to the same value eachtime, although in practice, it is recognized that this will not be thecase. However, if a hole or fluid leak exists in the fluid path then thepressure will not rise to the predetermined level and thus the line 126bsignal will not reach a comparison threshold voltage level establishedin circuit 128. The circuit is 128 will thus provide an output signal online 128b which indicates that the threshold has not been reached andthat a leak exists in the fluid path.

The static pressure and thus the threshold needed in circuit 128 willnot be the same for every instrument. Rather, it is a function of thetubing length, tubing diameters, and mechanical tolerances of each ofthe system components, etc . . . . Thus, a calibration step is used toset the threshold.

To calibrate the system, steps 1-5 are repeated as above. The sampleprobe is completely occluded, such as by placing a calibration tiphaving a closed end on the probe body. This establishes a calibrationvoltage for the comparison threshold of circuit 128. The actualthreshold voltage in circuit 128 is set a small voltage below that toinsure that the voltage on line 126b exceeds the threshold where leaksare not present and the output 128b changes to reflect that.

The level sense detect circuit 130 responds to the line 126b signal andcompares it to an internal reference. The output on line 130b is highuntil the disposable tip contacts a sample fluid. Then, with the airpump continuing to provide an air flow resulting in the pressuretransducer 122 causing a signal rise on line 126b above the thresholdvoltage the signal on line 130b drops typically to about zero volts,thus indicating that physical contact between the sample probe and thesample fluid has occurred.

The aspirate integrity circuit 132 receives the signal on line 126b andif it is below a threshold voltage level established internally providesan output signal on line 132b having a high voltage (typically 5 volts).Once the signal level on line 126b reaches the threshold voltage, thecircuit 132b provides on line 182b a low voltage (typically zero).

During an aspiration operation, the vent port of the pump valve isopened and the sample probe port of the pump valve should be closed tothus isolate the bleed valve, the coil and air pump from the dilutor andpressure transducer. However, it is not possible to determine whetherthe pump valve operated correctly. Thus, with the sample probe port ofthe pump value closed, air is aspirated through the sample probe. Thisshould result in a pressure change in the fluid path in which thetransducer is disposed.

If a leak exists in the pump valve, however, the pressure generated dueto the aspirate operation will not rise to the proper threshold levelfor circuit 132. Consequently the pressure transducer 122 would providea signal having an amplitude insufficient to reach the thresholdvoltage. The circuit 132 thus provides at the output terminal 132b asignal having a voltage level indicating that a leak in the pump valvewas detected during an aspirate operation.

The clot detection circuit 134 has a dual comparison function with apair of threshold levels set to detect whether the voltage level of thesignal on line 126b falls within a predetermined voltage range betweenthem. This is because the pressure transducer 122 (FIG. 3) measuresdifferent pressure levels during aspirate and dispense operations. Forexample, when the dilutor piston stops after being moved during anaspirate operation, the pressure measured by the transducer should dropbelow a predetermined threshold voltage. If the pressure transducerfails to indicate such a pressure drop, then the voltage level of theline 126b signal would likewise not change, thus indicating that thesample probe tip was occluded.

Similarly, during a dispense operation the pressure should stay above apredetermined threshold voltage. If the pressure transducer senses apressure drop or rise during a dispense operation, then the voltagelevel of the line 126b signal would likewise change to a level outsidethe predetermined threshold voltage range thus indicating that thesample probe tip was occluded during the dispense operation.

The clot detection circuit may also be of the type described inco-pending patent application Ser. No. 08/826,330, filed Mar. 27, 1997,now U.S. Pat. No. 5,777,221 assigned to the assignee of the presentinvention and incorporated herein by reference.

The pump servo integrity circuit 138 with input terminal is coupled toan output terminal, line 126c, of the sample and hold circuit 174 (FIG.4). If this air pump servo voltage signal is not within a predeterminedrange, then the pump servo integrity circuit 138 provides an outputsignal so indicating at output terminal 138b. This signal is onlyexamined by the controller 94 when setting the pump voltage.

Two threshold voltages are provided in circuit 138 respectively set atopposite voltage extremes. The threshold voltage levels are selectedsuch that if the line 126c signal level exceeds these threshold levels,it indicates that the pump control circuit is unable to servo the pumpin the desired manner. Thus, when the line 126c signal is between thesethresholds, the line 138c output voltage level is high. When the line126c signal is outside the threshold voltage range the output signal isabout zero volts.

It should be noted, that in some embodiments, it may be preferable todetect the signal level of the line 126b signal rather than the line126c signal in which case the threshold voltage levels would be setdifferently.

The tip detection circuit 136 has a pair of threshold levels setinternally at +/- low voltages defining a range about zero for the line126b signal level above which the line 126b signal goes upon tipinstallation and below which line 126b goes on tip removal.

Inside this range, the output 136b is high; outside the range the outputis low.

When a disposable sample tip is placed on the sample probe body, thevoltage level of the line 126b signal will rapidly rise. The controller94 examines the signal level of line 136b at the output terminal of thetip detection circuit. The controller 94 detects the signal level of theline 136b signal and verifies that the signal remains high for apredetermined time period, typically about ten msec. Then, there existsa relatively high probability that a tip was in the tip holder and thata disposable tip was actually placed onto the probe body.

In a similar manner, when a disposable tip is removed from the probebody, a pressure change occurs and is detected by the pressuretransducer. The pressure transducer provides a corresponding outputsignal having a voltage transient below the range which is detected. Inthe event that a tip is not removed, the controller 94 detects the line136b signal staying in range and acts to prevent a new tip from beingdisposed over an old tip that was not removed.

Referring now to FIG. 5, an additional embodiment of an aspirating anddispensing apparatus 66' is shown. In this embodiment, the constant airsource, damper coil, pump valve, and filter of an earlier describedembodiment have been eliminated. The aspirating and dispensing apparatus66' includes a bleed valve 80' which is controlled by a controller 94.In a particular embodiment, the controller 94 is provided as amicroprocessor based controller. Port 80b of bleed valve 80' is coupledto a first port 82a of a three port connecting member 82. Port 80a ofbleed valve 80' may be open to the atmosphere or include a filter 300. Asecond port 82b of the three port connecting member 82 is coupled to theoutput of a dilutor 84. A third port 82c of the three port connectingmember 82 is connected to a tube 96 having an inner diameter which fitsthe port 82c sealing the connection.

A pressure transducer 98 has a first port 98a coupled to a second end ofthe tube 96 and has a second port 98b coupled to a first end of a tube100. A second end of the tube 100 is coupled to a first port 106a of asample probe 102. Thus, the connecting member 82 and tubes 96, 100, andpressure transducer 98 provide a fluid path between the sample probe102, the bleed valve 80', and the dilutor 84.

To facilitate connecting of the transducer ports 98a, 98b to therespective ones of the tubes 96, 100 with substantially differentdiameters, each of the ports 98a, 98b has coupled 25 thereto a matingtube 101. The mating tubes 101 are provided from a relatively flexiblematerial having a relatively high elasticity characteristic and anon-stretched diameter selected to accept the outside diameter of thetubes 96, 100 with a slight interference fit.

The sample probe 102 includes a probe body 106 having a channel 110between a first fluid port 106a to which the system tubing 100 iscoupled and having a second fluid port 106b to which a sample probe tip108 is coupled.

The tube 100 which couples the transducer 98 to the sample probe 102 ishere provided having a length typically of about nine and one-halfinches and is comprised of a generally resilient material. It isdesirable to minimize the distance between the sample probe 102 and thepressure transducer 98. In some applications, it may be desirable oreven necessary to place the pressure transducer 98 closer than nine andone-half inches from the sample probe 102 and as close as possible tothe sample probe 102.

In this particular embodiment, the flow through pressure transducer 98has a pair of electrical terminals 98c and 98d, one of which correspondsto a positive output terminal and one of which corresponds to a negativeoutput terminal of the transducer 98. The transducer 98 provides adifferential output voltage on the output terminals 98c and 98drepresentative of the pressure difference between the pressure measuredin the sample probe tip 108 and an ambient atmospheric pressure.

The transducer 98 is electrically coupled through lines 111 to adetector circuit 112 at a pair of input terminals 112a and 112b. Thedetector circuit 112 receives input signals from the pressure transducer112 and provides at its output terminals 112c and 112d output signals tocontroller 94.

In operation, and as shown in the flow chart of FIG. 6, the system 66'operates as follows. The two port bleed valve 80' is opened and a firstvolume of air is aspirated by dilutor 84 through ports 80a and 80b ofbleed valve 80' as recited in step 210 of method 200. A shaft 90 couplesa piston 86 to a linear stepper motor 92. In response to signalsreceived from controller 94, the stepper motor 92 drives the piston 86in first and second opposite directions within the housing 88. In apreferred embodiment, the controller 94 is provided as a microprocessorbased controller. In a particular embodiment, fifty microliters of airare aspirated, though other embodiments could aspirate different volumesof air.

At step 220, the controller 94 closes ports 80a and 80b of the bleedvalve 80'. Step 230 recites that air is dispensed by activation ofdilutor 84, while the probe 108 is being moved toward the fluid 32. Inthis particular embodiment, the dispense rate is approximatelythirty-seven and one-half microliters per second, though otherembodiments could utilize different dispense rates. This aspiration ofair through the probe results in a positive pressure measured by thepressure sensor 98 in step 240. This measured pressure value is used asa reference. As shown next in step 250, the pressure is monitored, andonce sample probe tip 108 enters the fluid, the pressure measured by thepressure sensor 98 increases as recited in step 260. The differencebetween the current pressure and the reference pressure is compared to apredetemined threshold value to indicate the detection of the fluidlevel. At step 270, upon detection of the fluid level, the movement ofthe sample probe 102 is stopped, bleed valve 80' has ports 80a and 80bopened which will bleed the pressurized air within the system into theatmosphere. Also, at this time, dilutor 84 is repositioned to its homeposition (i.e. the fully dispensed position). Next, as shown in step280, a volume of air (approximately two microliters in this embodiment)is aspirated by dilutor 84 through ports 80a and 80b of bleed valve 80'to remove any dilutor backlash. As recited in steps 285 and 290, afterapproximately two hundred milliseconds bleed valve ports 80a and 80b areclosed. This waiting period allows the system to finish bleedingpressurized air back to the atmosphere. As shown in step 295, the fluidcan now be aspirated.

The aspiration and dispensing apparatus described above is capable ofaccurately dispensing and aspirating various quantities of fluid and ismost particularly suited for accurately aspirating and dispensing tenmicroliter through two hundred microliter volumes of fluid, such as airor a sample liquid. Since the present invention dispenses air out of thesample probe, instead of aspirating air into the probe, none of theliquid is aspirated into the sample probe upon detection of the liquidsurface. It is only after the liquid surface has been determined thataspiration of the liquid may be commenced. Thus, the present inventionprovides the advantage of producing a more accurate aspiration of liquidsince none of the liquid is aspirated upon detection of the surface ofthe liquid. This advantage is particularly critical whendetecting/aspirating small levels of liquid. Further, an additionalbenefit provided by dispensing air out of the sample probe is that thedilutor which is providing the aspiration is closer to its homeposition, thus, the aspiration of the liquid can occur more quicklywhich decreases the cycle time, resulting in higher system throughput.

Having described preferred embodiments of the invention it will nowbecome apparent to those of ordinary skill in the art that otherembodiments incorporating these concepts may also be used. Accordingly,it is submitted that the invention should not be limited to thedescribed embodiments but only by the spirit and scope of the appendedclaims.

What is claimed is:
 1. An apparatus for aspirating and dispensing asample fluid comprising:a dilutor having a port; a bleed valve having afirst port and a second port; a flow through pressure transducer, havinga first port and a second port, said transducer operative to provide apressure signal; a connecting member having a first port, a second port,and a third port in mutual fluid communication, wherein the first portis coupled to the port of said dilutor, the second port is coupled tothe first port of said bleed valve, and the third port is coupled to thefirst port of said flow through pressure transducer; a sample probehaving a first port coupled to the second port of said flow throughpressure transducer; and a controller in communication with saiddilutor, said bleed valve, and said flow through pressure transducer,said controller operative to control said dilutor and said bleed valvein response to said pressure signal.
 2. The apparatus of claim 1 whereinsaid flow through pressure transducer comprises a pair of electricaloutput terminals, and wherein said flow through pressure transducer isoperative to provide said pressure signal as a differential outputsignal on said pair of electrical output terminals in response topressure changes in a fluid path between said sample probe and saidconnecting member.
 3. The apparatus of claim 1 wherein said controllercomprises a microprocessor.
 4. The apparatus of claim 1 furthercomprising a detector circuit for receiving said pressure signal fromsaid flow through pressure transducer and for providing said controllerwith a signal reflecting one of a plurality of conditions represented bythe pressure in said flow through pressure transducer.
 5. The apparatusof claim 4 wherein said detector circuit comprises:an amplifier circuithaving a first terminal coupled to said flow through pressure transducerand having a second terminal; a signal conditioning circuit having afirst terminal coupled to the second terminal of said amplifier circuitand having a second terminal coupled to at least one of: (a) a fluidlevel detector circuit; (b) an aspirate integrity circuit; (c) a clotdetector circuit; and (d) a tip detector circuit, each of which having anominal state signal set therein by said signal conditioning circuitunder operation of said controller.
 6. The apparatus of claim 5 whereinsaid signal conditioning circuit comprises:a first inverting amplifierhaving a negative input terminal coupled to the first terminal of saidsignal conditioning circuit, having a positive input terminal and havingan output terminal; a second inverting amplifier having a negative inputterminal coupled to the output terminal of said first invertingamplifier, having a positive input terminal coupled to ground and havingan output terminal; and a sample and hold circuit having an inputterminal coupled to the output terminal of said second invertingamplifier, having an output terminal coupled to the positive inputterminal of said first inverting amplifier and having a controlterminal, wherein said controller is coupled to the control terminal ofsaid sample and hold circuit.
 7. A method for aspirating and dispensinga sample fluid comprising the steps of:aspirating a first volume of airwith a dilutor; dispensing with the dilutor at least a portion of saidfirst volume of air through a sample probe in fluid communication withthe dilutor while moving the sample probe towards a target sample fluidvolume; measuring the pressure in the sample probe using a transducer;storing a value of the measured pressure in a detection circuit;monitoring the pressure in the sample probe using said transducer;detecting, with said detection circuit, a pressure increase as thesample probe enters the sample fluid volume; comparing, using thedetection circuit, the difference between the stored pressure value andthe currently monitored pressure to a threshold value to determine thedetection of the sample fluid; stopping movement of the sample probeupon detection of the pressure increase as the sample probe enters thesample fluid; bleeding pressurized air from the sample probe whilereturning the dilutor to a home position; aspirating a second volume ofair with the dilutor; waiting a predetermined period of time; andaspirating said sample fluid with the dilutor.
 8. The method of claim 7wherein the step of aspirating a first volume of air comprisesaspirating through a bleed valve in fluid communication with thedilutor.
 9. The method of claim 7 wherein the step of aspirating a firstvolume of air comprises aspirating approximately fifty microliters ofair.
 10. The method of claim 7 wherein said step of dispensing furthercomprises dispensing at a rate of approximately thirty-seven andone-half microliters per second.
 11. The method of claim 7 wherein thestep of bleeding further comprises bleeding through a bleed valve influid communication with the sample probe and the dilutor.
 12. Themethod of claim 7 wherein the step of aspirating a second volume of aircomprises aspirating through a bleed valve in fluid communication withthe dilutor.
 13. The method of claim 7 wherein the step of aspirating asecond volume of air comprises aspirating approximately two microlitersof air.
 14. The method of claim 7 wherein the step of waiting apredetermined period of time comprises waiting approximately two-hundredmilliseconds.